This invention relates to a fluid displacement apparatus, and more particularly, to a scroll type fluid displacement apparatus for use as a supercharger for an engine or as an air pump.
Scroll type fluid displacement apparatus are well known in the prior art. For example, U.S. Pat. No. 801,182 (Creux) discloses a fluid displacement device including two scrolls, each having a circular end plate and a spiroidal or involute spiral element. These scrolls are maintained angularly and radially offset so that both spiral elements interfit to make a plurality of line contacts between their spiral curved surfaces to thereby seal off and define at least one pair of fluid pockets. The relative orbital motion of the scrolls shifts the line contacts along the spiral curved surfaces, and as a result, the volume of the fluid pockets changes. Since the volume of the fluid pockets increases or decreases dependent on the direction of the orbital motion, the scroll type fluid apparatus is applicable to compress, expand or pump fluids.
Scroll type fluid displacement apparatus have been used as refrigeration compressors in refrigerators or air conditioners. Such compressors need high efficiency and a high compression ratio, such as a 5 to 10 compression ratio. In such a compressor, the re-expansion volume, i.e., the smallest volume of the fluid pockets in the compression cycle, which is located at the center of the scrolls, must be reduced as much as possible. To this end, the inner end portions of the spiral elements are extended inwardly as far as possible to the center of the scroll.
The conventional driving mechanism in a high compression ratio scroll type compressor is connected to the end plate of an orbiting scroll on a side opposite the spiral element. The acting point of the driving force of the driving mechanism on the orbiting scroll generally is displaced from the acting point of the reaction force of the compressed gas, which acts at an intermediate location along the height of the spiral element of the orbiting scroll. If the distance between these acting points is relatively long, a moment is created which adversely effects the stability of the orbiting scroll during orbital motion. Therefore, to compensate for this loss of stability, the length of the spiral element generally is limited, which in turn limits the volume of the apparatus.
The above limitation on the length of the spiral element is not a problem for a scroll type fluid displacement apparatus which requires a compression ratio of only 1.0 to 1.5, since the re-expansion volume need not be reduced as much as in a high compression ratio apparatus. In an apparatus which requires only a low compression ratio, the difference in pressure between the high pressure space and the lower pressure space is smaller than in a high compression ratio apparatus, so that 1.5 to 2.0 revolutions of the spiral element generally is sufficient.
A scroll type fluid displacement apparatus generally has a balanceweight to cancel the dynamic imbalance caused by the centrifugal force of the orbital moving parts. The balanceweight is usually located on the drive shaft, so that the apparatus must have space to rotatably enclose the balanceweight within the apparatus. Therefore, the axial and radial dimensions of the apparatus are increased.